a) Field of the Invention
The present invention relates to a surface illuminant device used for a backlight of a liquid crystal display or the like.
b) Description of the Prior Art
FIG. 1 shows a conventional surface illuminant device using a light transmitter, in which linear light sources 1 are disposed adjacent to the ends of a light transmitter 2, and a diffusing member 3 and a reflecting member a are situated on the front and back of the light transmitter 2, respectively.
In this surface illuminant device, light from the light sources 1 is incident through entrance ends 2a on the light transmitter 2 and transmitted. By forming a rough surface or fine convex and concave parts on the back of the light transmitter 2 to change the orientation of the light reflected from the back and the reflecting member 4, part of the light transmitted through the light transmitter 2 emerges from a surface 2b of the light transmitter 2. The light emerging from the light transmitter passes through the diffusing member 3 and becomes diffused light, which illuminates a liquid crystal display panel 5, for example.
FIG. 2 shows another conventional surface illuminant device, which uses a reflecting member. In this surface illuminant device, the linear light sources 1 are disposed inside a reflecting member 6 of sectional shape such as that shown in the figure, and light emerging directly from the light sources 1 or after being reflected by the reflecting member 6 is incident as diffused light through the diffusing member 3 on the liquid crystal display panel 5, which is illuminated.
The conventional surface illuminant devices mentioned above are designed to provide uniform illumination through the diffusing member 3, no matter whether the light transmitter shown in Fig. 1 is used or the reflecting member in FIG. 2 is used. This brings about divergent light, which fails to enter the liquid crystal display panel 5, and a loss of light, making it difficult to provide bright illumination. In particular, the light incident on the liquid crystal display panel 5 is diffused light following all directions, and when an observer makes observations, for example, from a direction perpendicular to the liquid crystal display panel, some of the light does not enter his eye and thus a bright image can not be observed.
In order to eliminate these defects, an arrangement is known in which a prism sheet 7 having a prism surface with prisms 7a arranged as shown in FIG. 3 is situated between the diffusing member 3 and the liquid crystal panel 5, and the diffused light from the diffusing member 3 is directed as normal to the liquid crystal display panel 5 as possible, thereby allowing a bright image to be viewed.
Where the prism sheet 7 shown in FIG. 3, as mentioned above, is disposed in front of the surface illuminant device and the liquid crystal display panel 5 is illuminated through the prism sheet, brightness varies with orientation of the longitudinal direction of each prism 7a of the prism sheet 7, and therefore the direction of its ridge line. Thus, only relatively dark illumination is sometimes obtained.